MEMPHIS — If you are a Verizon customer on the East Coast, odds are good that your cellphone or tablet arrived by way of a beige, windowless warehouse near Tennessee’s border with Mississippi.

Inside, hundreds of workers, many of them women, lift and drag boxes weighing up to 45 pounds, filled with iPhones and other gadgets. There is no air-conditioning on the floor of the warehouse, which is owned and operated by a contractor. Temperatures there can rise past 100 degrees. Workers often faint, according to interviews with 20 current and former employees.

One evening in January 2014, after eight hours of lifting, Erica Hayes ran to the bathroom. Blood drenched her jeans.

She was 23 and in the second trimester of her first pregnancy. She had spent much of the week hoisting the warehouse’s largest boxes from one conveyor belt to the next. Ever since she learned she was pregnant, she had been begging her supervisor to let her work with lighter boxes, she said in an interview. She said her boss repeatedly said no.

She fainted on her way out of the bathroom that day. The baby growing inside of her, the one she had secretly hoped was a girl, was gone.

“It was the worst thing I have ever experienced in my life,” Ms. Hayes said.

Three other women in the warehouse also had miscarriages in 2014, when it was owned by a contractor called New Breed Logistics. Later that year, a larger company, XPO Logistics, bought New Breed and the warehouse. The problems continued. Another woman miscarried there this summer. Then, in August, Ceeadria Walker did, too.

The women had all asked for light duty. Three said they brought in doctors’ notes recommending less taxing workloads and shorter shifts. They said supervisors disregarded the letters.

Pregnancy discrimination is widespread in corporate America. Some employers deny expecting mothers promotions or pay raises; others fire them before they can take maternity leave. But for women who work in physically demanding jobs, pregnancy discrimination often can come with even higher stakes.

The New York Times reviewed thousands of pages of court and other public records involving workers who said they had suffered miscarriages, gone into premature labor or, in one case, had a stillborn baby after their employers rejected their pleas for assistance — a break from flipping heavy mattresses, lugging large boxes and pushing loaded carts.

They worked at a hospital, a post office, an airport, a grocery store, a prison, a fire department, a restaurant, a pharmaceutical company and several hotels.

But refusing to accommodate pregnant women is often completely legal. Under federal law, companies don’t necessarily have to adjust pregnant women’s jobs, even when lighter work is available and their doctors send letters urging a reprieve.

The Pregnancy Discrimination Act is the only federal law aimed at protecting expecting mothers at work. It is four paragraphs long and 40 years old. It says that a company has to accommodate pregnant workers’ requests only if it is already doing so for other employees who are “similar in their ability or inability to work.”

That means that companies that do not give anyone a break have no obligation to do so for pregnant women. Employees say that is how the warehouse’s current owner, XPO Logistics, operates.

For example, last October, a 58-year-old woman died of cardiac arrest on the warehouse floor after complaining to colleagues that she felt sick, according to a police report and current and former XPO employees. In Facebook posts at the time and in recent interviews, employees said supervisors told them to keep working as the woman lay dead.

If companies “treat their nonpregnant employees terribly, they have every right to treat their pregnant employees terribly as well,” said Representative Jerrold Nadler, Democrat of New York, who has pushed for stronger federal protections for expecting mothers.

In every congressional session since 2012, a group of lawmakers has introduced a bill that would do for pregnant women what the Americans With Disabilities Act does for disabled people: require employers to accommodate those whose health depends on it. The legislation has never had a hearing.

“We are deeply troubled by these allegations,” said a Verizon spokesman, Rich Young. “We have no tolerance — zero tolerance — for this sort of alleged behavior.” He said the company opened an internal investigation in response to The Times’s inquiry. “None of these allegations are consistent with our values or the expectations and demands of contractors that work directly for us or have any affiliation with us.”

Erin Kurtz, an XPO spokeswoman, said: “We’re surprised by the allegations of conduct that either predate XPO’s acquisition of the Memphis facility or weren’t reported to management after we acquired it in 2014.” She said the allegations “are unsubstantiated, filled with inaccuracies and do not reflect the way in which our Memphis facility operates.” The company also disputed that the warehouse was windowless, noting that there were a number of interior windows.

Ms. Kurtz said XPO prioritized the safety of its workers, had “no tolerance for any type of discriminatory behavior” and has enhanced pay and benefits for employees in recent years.

Those improvements didn’t help Ceeadria Walker when she got pregnant. The 19-year-old said she gave her XPO supervisor a doctor’s letter from OB/GYN Centers of Memphis saying she should not lift more than 15 pounds. She said she asked to be assigned to an area with lighter items. Ms. Walker said her supervisor regularly sent her to a conveyor belt line where she had to lift more than she was supposed to. She miscarried the day after spending her shift handling those heavier boxes.

A pregnant Ceeadria Walker continued lifting heavy boxes at XPO after being denied a reprieve. The day after a grueling shift in August, she miscarried.

Miranda Barnes for The New York Times

“We’re saddened that Ms. Walker had a miscarriage over the summer,” Ms. Kurtz said. “We’re investigating these newly raised claims.”

The Risks of Lifting

For most women, it is safe to work while pregnant.

But there is “a slight to modest increased risk of miscarriage” for women who do extensive lifting in their jobs, according to guidelines published this year by the American College of Obstetricians and Gynecologists. The recommendations are intended to inform doctors about best practices.

Two decades of medical research have established a link between physically demanding work and fetal death, though there is debate about how strong the connection is. Part of the difficulty in measuring the relationship, researchers say, is that it’s impossible to design a study that isolates the impact of heavy lifting versus other risk factors, like pre-existing conditions.

In a peer-reviewed study from 2013, researchers in Denmark found that the risk of fetal death increased as women lifted heavier objects more frequently. The researchers theorized that lifting and bending could reduce blood flow to the uterus.

Another possibility, doctors said, is that extreme physical exertion diverts blood from a woman’s womb to her muscles.

The potential dangers are greatest for women whose pregnancies are already classified as high risk, which is why doctors often advise that they be given easier tasks.

“When employers ignore these medical recommendations, they are potentially jeopardizing patients’ health,” said Rebecca Jackson, the chief of obstetrics and gynecology at San Francisco General Hospital. “It’s especially bothersome to me that this is occurring for women in strenuous jobs, given that they are at the most risk of injuring themselves or the pregnancy.”

Warehouses are among the fastest growing workplaces in the country, employing more than a million Americans. Retailers, competing against the likes of Amazon, demand high speed at low costs.

On Memphis’s east side, these are often the highest-paying jobs available for people without college degrees. Drawn by the proximity to rail lines and highways, some of the country’s largest companies have set up distribution centers here. One dispatches Nike shoes. Another handles Disney toys. And a short drive from Graceland, Verizon has its hub.

XPO runs all of those warehouses. The Verizon facility, which XPO took over when it bought New Breed Logistics in 2014, is the only one where The Times interviewed workers about pregnancy discrimination. Shifts there can last 12 hours. Workers get 30 minutes for lunch and as many as three other 15-minute breaks.

XPO’s 2017 employee handbook warns that taking unapproved breaks, arriving to work late or leaving early can result in “immediate termination,” unless the reasons for the departures are “legally protected.” The Pregnancy Discrimination Act does not guarantee women such protections.

A Job on the Line

It was the fall of 2013, and Erica Hayes was convinced that she was having a girl. She daydreamed about the clothes she would buy and made a list of favorite names. Her friend was pregnant with a boy, and they talked about raising their children together.

At first, Ms. Hayes was processing individual shipments to Verizon customers — one phone, one charger, onto the next. Then, a crush of holiday orders hit the warehouse in December. She said that her boss began dispatching her to the area of the warehouse that handled bulk shipments, often destined for Verizon stores, where the warehouse was struggling to keep up. She often spent up to 12 hours a day lifting huge boxes, some with 20 iPads and 20 accessories.

She said she could have handled paperwork or stayed in the section of the warehouse devoted to small shipments. But she said her supervisor kept ordering her to work with the largest boxes. Ms. Hayes’s mother said that her daughter talked to her about the rejected requests at the time.

Ms. Hayes said she began to bleed regularly at work. She sometimes left early to go to the hospital. Each time, she said, her supervisor wrote her up. As the demerits accumulated, she stopped leaving. Instead, she bled through four maxi pads a day.

“My job was on the line,” she said. At the end of a long shift in January 2014, she felt blood gushing into her jeans.

A co-worker fetched her a black peacoat to wrap around her waist to cover the spreading stain. Another grabbed plastic bags to line the leather driver’s seat of her 2003 Hyundai. Ms. Hayes fainted before she could get to the car. An ambulance took her to the hospital.

A couple of weeks later, she said, her supervisor handed her a $300 invoice for the cost of the ambulance ride. (Ms. Hayes, who still works at the warehouse and is hoping for a promotion, said she never paid the bill.)

That spring, two more women had miscarriages at the warehouse. Both said that their supervisors rejected their requests to pack lighter boxes.

One of the women, who still works at the warehouse, declined to be identified for fear of losing her job.

The other was Tasha Murrell. She already had two boys and was praying for a girl. She planned to name the baby Dallas, after the Cowboys, her favorite football team. Ms. Murrell said that she told her boss she was pregnant and asked to leave work early one day that spring because the lifting had become painful.

While employed at the Verizon warehouse in 2014, Tasha Murrell, pregnant at the time, told a supervisor she was in pain and asked to leave early; the manager said no. Ms. Murrell miscarried the next day.

Miranda Barnes for The New York Times

Ms. Murrell’s husband keeps the ultrasound from the pregnancy she lost in his wallet. It has become deeply creased from being in his wallet.

Miranda Barnes for The New York Times

Her supervisor told her to get an abortion, according to a discrimination complaint she filed with the Equal Employment Opportunity Commission in April 2018. Ms. Murrell woke up the next morning to find her mattress stained with blood. Her husband drove her to the emergency room, where doctors told her she had miscarried. Ms. Murrell left the job last year and is now an organizer with the Teamsters, which is trying to organize a union at the warehouse.

Ms. Hayes and Ms. Murrell had the same supervisor: Amela Bukvic. Through her lawyer, Ms. Bukvic denied telling anyone to get an abortion. “I would never make such a horrible statement to anyone, especially an employee under my supervision,” she said.

Ms. Bukvic said that she made sure that the pregnant women whom she managed had workloads that were not excessive. She said she never denied help to the pregnant employees. “If they had any work restrictions, I always took all steps to make sure their work duties never exceeded those restrictions,” she said.

A few months later, in September 2014, it happened to another woman.

Chasisty Bee, 33, was four months pregnant. Hoping for a girl, she bought a newborn’s blanket from Burlington Coat Factory.

Ms. Bee had miscarried in 2008 while working at the Verizon warehouse. This time, she said, she brought in a doctor’s note recommending that she work shorter shifts, be given a chair and light duty. Supervisors rejected her requests. One afternoon, after almost 14 hours on her feet, she started feeling dizzy and crumpled to the warehouse floor. Her physician told her that she had miscarried.

After Ms. Bee got pregnant again in 2015, she found a new job. “I couldn’t bear to lose another child,” she said. The next February, she gave birth to a healthy girl.

Ms. Kurtz, the XPO spokeswoman, said: “The false and misleading allegations directed at our Memphis facility are fueled by the Teamsters and are part of their ongoing, but unsuccessful, attempts at organizing.”

Stalled Momentum

A bipartisan group of lawmakers has proposed upgrading the 1978 Pregnancy Discrimination Act. The bill would compel companies to accommodate pregnant women — for example, by offering extra breaks or the option of light duty — as long as it does not impose an “undue hardship” on their business. That is the same language used in the Americans With Disabilities Act.

Women “shouldn’t have to choose between keeping a doctor appointment or their job,” said Senator Dean Heller, Republican of Nevada, one of 125 co-sponsors of the Pregnant Workers Fairness Act in the House and Senate.

In 2015, it looked as if the bill might gain traction. The Supreme Court had just ruled in favor of Peggy Young, a UPS driver who was denied light duty after getting pregnant. Pregnancy discrimination was suddenly grabbing headlines.

But some Republicans, including Senator Lamar Alexander of Tennessee, where the XPO warehouse is, viewed that bill as adding a confusing new layer of regulations, according to Senate aides. Mr. Alexander, who is chairman of the Senate committee on health and labor, co-sponsored a competing bill. It expanded protections for pregnant women in some cases. But it still allowed employers to deny accommodations if they weren’t being provided to other workers in similar situations.

“It was a useful mechanism in order to divert some of the momentum that was building,” said Emily Martin of the National Women’s Law Center, an advocacy group focused on women’s rights.

Both bills stalled.

Outside Washington, there have been fewer roadblocks. At least 23 states have passed laws that are stronger than current federal protections (Tennessee is not among them). In Utah, Delaware, Colorado and New York, Republicans led the charge. In Nebraska, an anti-abortion Democrat pushed the measure.

“Women have lost their children due to the lack of robust pregnancy protections in the workplace,” said Catherine Glenn Foster, the president of Americans United for Life, an anti-abortion group. “Anyone who can’t get behind this or uses it as a political game — it’s a travesty.”

XPO Logistics has had run-ins with regulators over labor issues. Last year, the state of California awarded four truck drivers $855,000 in back wages after XPO misclassified them as independent contractors.

‘I Watched Her Die’

At the Albertsons grocery store in Atascadero, Calif., Reyna Garcia had one of the toughest jobs. She pushed 200-pound carts, dragged sacks of cat litter and climbed 10-foot ladders to stock goods.

Ms. Garcia got pregnant in July 2012, found out she was having a girl and decided to name the baby Jade.

Ms. Garcia told her boss that her pregnancy was high risk — she had previously given birth prematurely. She presented a doctor’s note saying she should not lift more than 15 pounds. The boss ignored the recommendation, according to a lawsuit she filed against Albertsons in federal court in Los Angeles.

“She was feeling like she wasn’t getting any response from her supervisor,” her doctor, Mareeni Stanislaus, said in an interview. She said the restrictions were “even more important” because Ms. Garcia had a high-risk pregnancy. Heavy lifting can prompt smooth muscles like the uterus to contract, potentially inducing preterm labor, Dr. Stanislaus said.

The regular twisting and hoisting caused intense pain, but Ms. Garcia needed the paycheck and the health insurance. She requested any other position — in the Albertsons bakery or at the meat counter or as a fruit cutter or in the pharmacy or at the customer service desk.

Her boss turned her down, according to the lawsuit, which included corroborating statements from her colleagues.

Her doctors sent two more notes. “She should avoid prolonged standing without a break and should avoid excessive bending and reaching and balancing,” Dr. Stanislaus wrote. Copies of the letters were included in the suit.

About three weeks later, in the middle of her shift, Ms. Garcia began feeling “pelvic pressure,” according to her lawsuit. She asked her boss for permission to leave early; he gave her a long list of tasks that she needed to finish first.

Ms. Garcia ended up working overtime. By the time she got home, she could feel her amniotic sac bulging between her legs. It felt “like a balloon coming out of me,” she said in a sworn statement. She went to the emergency room. She could feel something scratching from inside; her doctor told her it was the baby’s fingernails. She was 20 weeks pregnant.

Five days later, Ms. Garcia gave birth to Jade. The baby lived less than 10 minutes.

“My husband and I watched her die,” Ms. Garcia said in her statement. “It was the most painful thing I have ever experienced.”

In 2014, Albertsons settled Ms. Garcia’s lawsuit for an undisclosed amount; the deal prohibited her from speaking publicly about what happened. “The company has a policy against pregnancy discrimination, and we accommodate employees with pregnancy-related disabilities in accordance with state and federal law,” said Christine Wilcox, an Albertsons spokeswoman.

Ms. Garcia’s boss demoted her when she returned to work.

“I lost my baby for this job,” Ms. Garcia said, according to her lawsuit. “Why didn’t you give me help when I was pregnant and asked for it?”

Hard to Breathe

Workers at the Memphis warehouse thought conditions might improve when XPO Logistics acquired New Breed in September 2014.

At the time, XPO and its chief executive, Bradley S. Jacobs, were on a buying spree. His strategy was simple: slash expenses and deliver quick profits. Today XPO, based in Greenwich, Conn., has a market value of $12 billion.

The company has had multiple run-ins with regulators. In one instance, the California Labor Commissioner’s Office last year awarded four XPO truck drivers a total of $855,000 after finding that the company had misclassified them as independent contractors.

In Memphis, XPO’s ownership changed the warehouse, but not the way workers had hoped. Over the next couple of years, pressure on employees intensified and working conditions deteriorated, according to the 20 current and former employees. Supervisors began demanding that they pack 120 boxes an hour instead of 60. Some bosses penalized employees for spending too much time in the bathroom or on breaks.

Plaques marking her years of service at the warehouse hang in Chasisty Bee’s home. She left the company after getting pregnant in 2015. After two previous miscarriages while there, “I couldn’t bear to lose another child,” she said.

Miranda Barnes for The New York Times

A spokeswoman for XPO Logistics, which processes orders for Verizon, said the allegations made by pregnant workers were “unsubstantiated, filled with inaccuracies and do not reflect the way in which our Memphis facility operates.”

Miranda Barnes for The New York Times

Summer temperatures inside the warehouse regularly exceeded 100 degrees, the current and former employees said. It was so hot and humid that workers sometimes found it hard to breathe.

Every week from June through September 2017, at least one worker collapsed on the warehouse floor, the employees said. Supervisors took to wheeling the sick workers to the break room, where they sometimes received an ice pop.

On the morning of Oct. 17, 2017, Linda Neal crashed to the warehouse floor, dead of cardiac arrest.

Ms. Neal, 58, had suffered from heart problems. She previously had complained to her son, Dean Turner, that XPO supervisors would not let her leave early when she was feeling unwell, Mr. Turner said in an interview. That day, she had told managers that she was short of breath and asked for an extra break, but her supervisor rejected the request, according to Lakeisha Nelson, who witnessed the exchange and still works there. When Mr. Turner arrived at the warehouse, he said, three of his mother’s colleagues told him the same thing.

Managers told workers to keep moving boxes as her body lay on the floor, according to four employees who were at the warehouse that day, as well as contemporaneous Facebookposts.

Ms. Kurtz, the XPO spokeswoman, said the company did allow workers to leave for the day after Ms. Neal died. She added that the Teamsters “continue to shamefully exploit the passing of our colleague through the spreading of false information” in their effort to organize at the warehouse.

That same month, Ceeadria Walker joined XPO. The pay was decent, about $12 an hour, an upgrade from her last job at a car-parts warehouse. At first, she didn’t mind the frequent 12-hour shifts. But she learned she was pregnant in June 2018 and started feeling nauseated and dizzy at the end of the long days.

Ms. Walker said she gave her supervisor a doctor’s note saying she should reduce the number of hours on her feet to avoid hurting herself or endangering her pregnancy. “They looked at it like it was nothing,” she said.

There were days when XPO let her work in the “pit,” dealing with the paperwork for orders, but she said she spent most of July on the conveyor-belt line hoisting 45-pound boxes.

Ms. Walker toyed with leaving. But, she said, “I couldn’t just quit my job.” She was near the end of her first trimester and needed money. She planned to name the baby, her first child, after her boyfriend — Octavia for a girl, Octavius for a boy.

By the end of her shift on Aug. 4, Ms. Walker said she had handled hundreds of Verizon boxes. When she woke up bleeding at 6 a.m. the next day, her mother drove her to the doctor.

Two days later, Ms. Walker spoke with The Times. She was distraught. “This was going to be my first.”